The present invention relates to the art of making polyolefin films, and, in particular, to a process for enhancing the physical characteristics of such film by controlling the crystalline growth during film formation.
Films made of polyolefins, such as polypropylene, polyethylene, polyethylene terephthalate or other such polymers are in great demand in both the industrial and consumer marketplace because of their great strength and toughness, especially when they have been biaxially stretched. However, such films possess certain characteristics which can result in a defective end product because of processing requirements. For example, uncontrolled crystalline growth which can occur during film formation may detract from the desired film product. Specifically, such uncontrolled crystalline growth can detract from the optic quality of the film as well as reduce the tensile strength of the film.
In the past it has been known to use high-speed tubular water-bath process for production of polypropylene, high-density polyethylene (HDPE), low-density polyethylene (LDPE), nylon and other polymers, especially since the production rate can be as much as two or three times greater than other methods, e.g., air blown or chill roll, in which production speed is held down by cooling requirements. The tubular water-bath process has combined the air-blown process with chill roll or conventional water-bath process for commercial production of film and, generally, includes drawing a bubble of molten film over a large-diameter mandrel from which airstreams blown therethrough extend the film to the desired diameter. When the designated orientation diameter is reached, the film is plunged into a cold-water bath. Another stream of cold water can be within the mandrel whereby the mandrel's metal skin is cooled. The cold water on one side of the extruded film and the water-cooled mandrel on the other greatly accelerate the cooling process.
Another process known as the Dow-Taga process teaches a downward extrusion of film from a tubular die, followed by inflation of the tubular bubble to obtain the required layflat width. The film bubble is first stabilized by an air ring about 200 mm below the die, after which the film is sized and cooled with a water ring located about 300 mm below the air ring. Once the film is formed it can be collapsed, such as by use of collapsing boards, dried and wound.
In all the known prior art processes, crystallization is relatively uncontrolled since a high degree of orientation is imparted during expansion while the film is at or above the glass transition temperature, Tg. Consequently, it is very difficult to impart different desired enhanced physical characteristics; thus, these processes have not been considered commercially significant.
As a result of the present invention, however, a tubular blown polyolefin film can be produced on a commercial scale with highly controlled crystallization whereby process-incurred defective physical characteristics such as those as set forth above can be overcome.